Water ¹⁷O longitudinal and transverse relaxation rates have been measured in aqueous solutions and mesophases of the non-ionic alkyl oligo(ethylene oxide) surfactants C₁₂E_n(n= 4, 5, 8) over wide ranges of temperature and concentration. In addition, two reference systems have been investigated: high molecular weight poly(ethylene oxide) and the dimer 1,2-dimethoxy ethane. The relaxation data are consistent with the following picture of the state of water in the headgroup shell of C₁₂E_n aggregates. The shell is compact, containing less than 5 and possibly as little as 2–3 water molecules per EO group. The shell exhibits substantial structural integrity; its water content is essentially invariant with respect to changes in concentration. The headgroup shell is highly dynamic, the rate of water rotation in the shell being reduced by at most a factor of 5 (at room temperature) as compared to bulk water.